SCN1A Mutation Mosaicism in a Family with Severe Myoclonic Epilepsy in Infancy


Address correspondence and reprint requests to Dr. M. Morimoto at Department of Pediatrics, Kyoto Prefectural University of Medicine, Kawaramachi-Hirokoji, Kamigyo, Kyoto 602–8566, Japan. E-mail: or Dr. K. Yamakawa at Laboratory for Neurogenetics, RIKEN Brain Science Institute, Hirosawa 2-1, Wako-shi, Saitama 351-0198, Japan. E-mail:


Summary: Purpose: To investigate the genetic background of familial severe myoclonic epilepsy in infancy (SMEI) cases.

Methods: We performed mutation analyses of the sodium-channel gene SCN1A in two Japanese brothers with clinical features of SMEI and their parents, who had no history of febrile and epileptic seizures.

Results: Each patient showed nucleotide changes (c.[730G>T; 735G>T; 736A>T]) in the coding exon 6 of SCN1A that led to a truncation of the channel protein. Their father showed no mutations, but their mother showed the same mutation in a subpopulation of lymphocytes.

Conclusions: The maternal mosaicism explains the identical SCN1A mutations in the two brothers. This highlights the importance of investigating parental mosaicism even in sporadic SMEI cases.